It is well known that an oxidized aromatic primary amine color developing agent formed by oxidation with exposed silver halide reacts with a coupler to form a dye such as an indophenol, an indoaniline, indamine, an azomethine, a phenoxazine, a phenazine and the like, thus forming a color image.
In order to form a magenta color image, a 5-pyrazolone type coupler, a cyanoacetophenone type coupler, an indazolone type coupler, a pyrazolobenzimidazole type coupler or a pyrazolotriazole type coupler is employed.
Magenta color image forming couplers which have been widely used in practice and on which various investigations have been made are generally 5-pyrazolones. It is known that dyes formed from 5-pyrazolone type couplers are excellent in fastness to heat and light but they have the undesirable absorption of yellow in the region around 430 nm which causes color turbidity.
In order to reduce the yellow component, a pyrazolobenzimidazole nucleus as described in British Pat. No. 1,047,612, an indazolone nucleus as described in U.S. Pat. No. 3,770,447 and a pyrazolotriazole nucleus as described in U.S. Pat. No. 3,725,067 have been proposed as a magenta color image forming coupler skeleton. However, the magenta couplers described in these patents are still insufficient since they provide only poor color images when they are mixed with a silver halide emulsion in the form of a dispersion in a hydrophilic protective colloid such as gelatin, they have a low solubility in an organic solvent having a high boiling point, they have some difficulties in synthesis thereof, or they have a relatively low coupling activity in conventional developing solutions.
The present inventors have carried out extensive investigations on a novel magenta color image forming couplers free from the subsidiary absorptions in the regions around 430 nm which is the most disadvantageous point in view of spectral absorption characteristics of a dye formed from 5-pyrazolone type magenta couplers. As a result, the present inventors have found a certain group of couplers which provide a color image without a subsidiary absorption in a shorter wavelength side of the main absorption and which have good fastness and which can be easily synthesized.